Is Martin's part of Giant food stores?

They are sister companies

Yes... But you can not use their bonus card at Giants...

Yes, Giant is the parent company.

Giant is union, Local 400

Martin's is a subsidiary of Giant which Wikipedia says is owned by Netherlands company Ahold Delhaize. They also own Food Lion and a similar supermarket chain in Indonesia called Super Indo...wonder if I could use my Martin's card at Super Indo ;)

Yes it is

Yep looked on the Website. Downloaded the app. Had Enough of WallyWorld. The have removed too much of the inexpensive stuff and mmm no store coupons. Better sales etc shopping here